1. Field of the Invention
The present invention relates to a data communication system and a time slot allocation method for conducting data communication between one base station and a plurality of terminal stations and, more particularly, to a data communication system and a time slot allocation method for efficiently allocating time slots in time-sharing communication from a terminal station to a base station.
2. Description of the Related Art
With a data communication system for conducting data communication between one base station and a plurality of terminal stations, data communication between the base station and the terminal stations is conventionally realized by allocation of time slots by polling. One of conventional data communication techniques of this kind, for example, is the art disclosed in Japanese Patent Laying-Open (Kokai) No. Hesei 10-242981.
FIG. 14 shows a schematic structure of a data communication system disclosed in the above literature. With reference to FIG. 14, a terminal station 110 and a base station 120 are connected through an up-link side transmission line 130 for transferring data from the terminal station 110 to the base station 120 (upward) and a down-link side transmission line 140 for transferring data from the base station 120 to the terminal station 110 (downward). Although FIG. 14 illustrates only one terminal station 110 for convenience"" sake, a plurality of terminal stations 110 are actually connected through the up-link side transmission line 130 and the down-link side transmission line 140.
The terminal station 110 includes a polling wait buffer 111 for holding polling wait information, a polling request generation unit 112 for detecting a buffer length of the polling wait buffer 111 and comparing the buffer length and a predetermined threshold value to generate a polling request according to a comparison result, and a multiplexing unit 113 for multiplexing output of the polling wait buffer 111 and output of the polling request generation unit 112 and sending the multiplexed output onto the up-link side transmission line 130 to transfer the output to the base station 120.
The base station 120 includes a polling request identification unit 121 for identifying a polling request from the terminal station 110 transferred through the transmission line 130, a minimum guaranteed polling generation unit 122 for generating minimum guaranteed polling, and a shared band control unit 123 responsive to output of the minimum guaranteed polling generation unit 122 and output of the polling request identification unit 121 for allotting a time slot to a free region of a minimum guaranteed polling according to the amount of polling wait information. Output of the shared band control unit 123 is transferred to each terminal station 110 through the down-link side transmission line 140 to conduct polling.
According to thus structured conventional data communication system, first, at the terminal station 110, a polling request is generated according to an information wait state of the polling wait buffer 111 and the generated polling request is notified to the base station 120 through the transmission line 130. Then, at the base station 120, the polling request sent from the terminal station 110 is analyzed to insert a polling pattern into a free region of the polling generated at the minimum guaranteed polling generation unit 122 according to the polling request. The polling pattern in question is notified to each terminal station 110 through the transmission line 140. Because a polling pattern is determined according to a polling request generated at the terminal station 110, allocation of time slots to each terminal station 110 can be efficiently conducted in the system as a whole.
The polling request generation unit 112 compares the amount of information queuing at the polling wait buffer 111 and a predetermined threshold value and notifies the base station 120 of information indicating whether the amount of the wait information in question is larger or smaller than the threshold value in question. Then, the base station 120 changes a polling pattern based on the information in question. The base station 120 is therefore allowed to change a polling pattern dynamically according to a polling request from the terminal station 110, so that allocation of time slots to the respective terminal stations 110 can be more efficiently conducted in the system as a whole.
The polling request generation unit 112 also compares the amount of information queuing at the polling wait buffer 111 and a plurality of threshold values and notifies the base station 120 of information indicating which threshold value the amount of wait information in question exceeds. Then, the base station 120 changes a polling pattern based on the information in question. The base station 120 is therefore allowed to set a polling pattern appropriately according to the amount of wait information.
The above-described conventional data communication system, however, requires a terminal station to monitor a state of a polling wait buffer, which burdens processing at the terminal station.
Another shortcoming is that even when a terminal station has none of data to be transmitted, minimum guaranteed polling and response thereto are conducted to wastefully consume time slots.
A further shortcoming is that a burst of data traffic can not be coped with because information notified from a terminal station to a base station together with a polling request only includes the amount of information queuing at a polling wait buffer.
A still further shortcoming is that since there is no method of transcendentally finding a relationship between the amount of information existing in a polling wait buffer and a time slot necessary for transferring the information amount in question, even if a time slot is allocated based on the amount of information existing in the polling wait buffer, the allocation is not always optimum.
Moreover, periodic analyses of polling information obtained from a base station by polling burdens processing at the base station.
An object of the present invention is to overcome the shortcomings of the above-described conventional techniques and provide a data communication system for appropriately and efficiently allocating time slots without the need of monitoring of a buffer at a terminal station and the need of polling by a base station.
According to the first aspect of the invention, a data communication system having one base station and a plurality of terminal stations for conducting data communication from the terminal stations to the base station as time-sharing communication, wherein
the terminal station comprises
a data buffer for storing data to be transmitted, and
time slot allocation request means for, when data is accumulated at the data buffer, periodically making a request for allocation of time slots to the base station until the accumulated data is all transmitted, and
the base station comprises
time slot allocation means for, when accepting a time slot allocation request from the terminal station, if a free time slot exists, allocating the free time slot to the terminal station and if not, making other the terminal station which has been already allocated a time slot release the allocated time slot and allocating the released time slot to the terminal station which has made the time slot allocation request,
the data buffer of the terminal station transmitting data using a time slot allocated by the time slot allocation means of the base station.
In the preferred construction, the time slot allocation means of the base station allocates time slots with a predetermined number of slots added thereto in response to continuous time slot allocation requests from the time slot allocation request means of the terminal stations.
In another preferred construction, the time slot allocation means of the base station increases the number of time slots to be allocated in response to continuous time slot allocation requests from the time slot allocation request means of the terminal stations by stages according to the number of requests.
In another preferred construction, the time slot allocation means of the base station, in response to continuous time slot allocation requests from the time slot allocation request means of the terminal stations, makes, for each request, the terminal station holding the largest number of time slots among the terminal stations holding more time slots than those held by the base station release a necessary number of time slots.
In another preferred construction, the time slot allocation means of the base station allocates time slots with a predetermined number of slots added thereto in response to continuous time slot allocation requests from the time slot allocation request means of the terminal stations, and
the time slot allocation means of the base station, in response to continuous time slot allocation requests from the time slot allocation request means of the terminal stations, makes, for each request, the terminal station holding the largest number of time slots among the terminal stations holding more time slots than those held by the base station release a necessary number of time slots.
In another preferred construction, the time slot allocation means of the base station increases the number of time slots to be allocated in response to continuous time slot allocation requests from the time slot allocation request means of the terminal stations by stages according to the number of requests, and
the time slot allocation means of the base station, in response to continuous time slot allocation requests from the time slot allocation request means of the terminal stations, makes, for each request, the terminal station holding the largest number of time slots among the terminal stations holding more time slots than those held by the base station release a necessary number of time slots.
According to the second aspect of the invention, in a data communication system having one base station and a plurality of terminal stations for conducting data communication from the terminal stations to the base station as time-sharing communication, a time slot allocation method of allocating time slots to the terminal stations, comprising the steps of:
when data is accumulated at the data buffer, periodically making a request for allocation of time slots by the terminal station to the base station until the accumulated data is all transmitted; and
when the base station accepts a time slot allocation request from the terminal station, if a free time slot exists, allocating the free time slot by the base station to the terminal station and if not, making other the terminal station which has been already allocated a time slot release the allocated time slot and allocating the released time slot to the terminal station which has made the time slot allocation request.
In the preferred construction, the base station allocates time slots with a predetermined number of slots added thereto in response to continuous time slot allocation requests from the terminal stations.
In another preferred construction, the base station increases the number of time slots to be allocated in response to continuous time slot allocation requests from the terminal stations by stages according to the number of requests.
In another preferred construction, the base station, in response to continuous time slot allocation requests from the terminal stations, makes, for each request, the terminal station holding the largest number of time slots among the terminal stations holding more time slots than those held by the base stations release a necessary number of time slots.
In another preferred construction, the base station allocates time slots with a predetermined number of slots added thereto in response to continuous time slot allocation requests from the terminal stations, and
the base station, in response to continuous time slot allocation requests from the terminal stations, makes, for each request, the terminal station holding the largest number of time slots among the terminal stations holding more time slots than those held by the base station release a necessary number of time slots.
In another preferred construction, the base station increases the number of time slots to be allocated in response to continuous time slot allocation requests from the terminal stations by stages according to the number of requests, and
the base station, in response to continuous time slot allocation requests from the terminal stations, makes, for each request, the terminal station holding the largest number of time slots among the terminal stations holding more time slots than those held by the base station release a necessary number of time slots.
According to another aspect of the invention, a computer readable memory storing a time slot allocation control program for controlling a data communication system having one base station and a plurality of terminal stations for conducting data communication from the terminal stations to the base station as time-sharing communication to allocate time slots to the terminal stations, the time slot allocation control program comprising the steps of:
controlling the terminal stations to, when data is accumulated at the data buffer, periodically make a request for allocation of time slots to the base station until the accumulated data is all transmitted, and
controlling the base station to, when the base station accepts a time slot allocation request from the terminal station, if a free time slot exists, allocate the free time slot to the terminal station and if not, make other the terminal station which has been already allocated a time slot release the allocated time slot and allocate the released time slot to the terminal station which has made the time slot allocation request.
Other objects, features and advantages of the present invention will become clear from the detailed description given herebelow.